I ran a short Ultimite Justice League that was PL 15-18 [Super and Bats were 18, Wonderwoman, Abin Sur (Green Lantern), Flash and Black Tom (original Shazam) were 17, Plastic Man was 16, Green Arrow and Warrior (Guy Gardner) was 15]
It was a lot of fun but I set it during an invasion by Darkseid, so they fought Dog Soldiers, Tanks, Huge Flying Battle Wagons and then the Furries (who where very powerful.. the highest one was PL14, but I max lots of stuff all over the place and they gave the players a REAL run for the money).
Darkseid was of course the big bad and the end and they won... by making a deal
Higher-level games are fun but can be all over the place. Batman had like 15 hero points and would pull all kinds of crazy stuff BUT it was a lot of fun. The problem was always balancing and challenging them.
Things that I changed... was how Vulnerability worked a bit. I allowed normal Attributes to still add to the save. If Martian Manhunter, PL17-18 with that many ranks of Protection or Amazing save, you are going to be fighting people with Damage bonuses between 15 and 20… so a DC30-35, rolling a +0 is a bit hard
Someday I will run it again BUT I would totally play in one